I did this for several years before covid. Typically if there was a vendor you liked, you could just show up and ask them about openings, if they didn't actively have signs posted for help. More will probably be looking for help in a few months, since many run a slower schedule or are off completely during winter. Check the vendor lists of market websites to see what looks interesting (do you want to do veggies? dairy? pre-made foods? etc) and reach out to them on social if they're off season.

In my experience the cash thing is rarer than it used to be 5+ years ago. I'm sure there are many vendors that still pay cash, but mine really formalized their business during the time I worked with them and everything was payroll/automated official by the time I stopped (for other reasons, day job became too much to also have a heavy side gig).

It was a great way to get to know a neighborhood and feel connected to the community. I made some great friends. And it was such a change from my desk job to do something physical and outdoors and learn about a completely different industry that was very mentally healthy for me. I may have left DC if I hadn't found this. I found it great for reasons beyond just extra pay.

The people I know were regulars at their local farmers markets and eventually they were asked by the operators of certain stalls if they were interested in picking up shifts. So I guess you could go to your local FM and ask around? Sorry that I can't be more helpful than that!